The present invention relates to a thin semiconductor film gas sensor device of the type comprising an insulating substrate, a thin semiconductor film applied to the substrate and a resistive heating element for heating the substrate and the semiconductor film to a predetermined operating temperature.
Sensor devices of the above mentioned type are very well known to experts in the trade and have been manufactured on a large scale since the nineteen seventies.
Initially, the sensor was made by depositing the film on a tube-shaped ceramic substrate and using as heating element a wire made of a high-melting metal.
According to recent manufacturing methods, improved sensor reproducibility can be obtained using an alumina substrate having, on one face, the heating element made of conductive materials, and on the other, the gas sensor film and the electrical contacts.
A full survey of sensor production methods based on sputter deposition (sputtering) of all the films constituting the sensor is provided by the references listed below, numbered 1-16:
[1] G. Sberveglieri et al., Sensors and Actuators B 4 (1991), pages 457-461, Elsevier Sequoia S. A., Lausanne;
[2] G. Sberveglieri et al., Sensors and Actuators B 5 (1991), pages 253-255, Elsevier Sequoia S. A., Lausanne;
[3] G. Sberveglieri et al., Journal of Materials Science Letters 10 (1991), pages 602-604, Chapman and Hall;
[4] G. Sberveglieri et al., Sensors and Actuators B 7 (1992), pages 721-726, Elsevier Sequoia;
[5] G. Sberveglieri, G. Faglia, S. Groppelli, P. Nelli, Tech. Digest 6th Int. Conf. Solid State Sensors and Actuators, San Francisco, Calif., USA (1991), pages 165-168;
[6] G. Sberveglieri, Sensors and Actuators B 6 (1992), pages 239-247, Elsevier Sequoia S.A.;
[7] G. Sberveglieri et al., Sensors and Actuators B 15-16 (1993), pages 86-89, Elsevier Sequoia S.A.;
[8] G. Sberveglieri, Abstract New Developments in Semiconducting Gas Sensors Sept. 13-14, 1993, Castro Marina (Italy);
[9] G. Sberveglieri, S. Groppelli, P. Nelli, Abstract Eurosensors VIII Sept. 25-28, 1994, Tolouse (France);
[10] G. Sberveglieri, Sensors and Actuators B 23 (1995), pages 103-109, Elsevier Science S.A.;
[11] G. Sberveglieri et al., Advanced Materials 8 No. 4 (1996), pages 334-337, VCH Verlagsgesellschaft mbH;
[12] M. Ferroni et al., Sensors and Actuators B 44 (1997), pages 499-502, Elsevier Science S.A.;
[13] G. Faglia et al., Sensors and Actuators B 57 (1999), pages 188-191, Elsevier Science S.A.;
[14] E. Comini et al., Sensors and Actuators B 68 (2000), pages 168-174, Elsevier Science S.A.;
[15] E. Comini et al., Sensors and Actuators 70 (2000), pages 108-114, Elsevier Science B.V.;
[16] E. Comini et al., J. Mater. Res., 16 No. 6 (2001), pages 1559-1564, Material Research Society.
In most cases, sensor film patterning is obtained using shadow mask technology.
FIGS. 1 to 7, relating to prior art, schematically illustrate the steps in the production of a double-sided sensor.
The overall process comprises two steps for the lower face, that is to say, depositing the pads (rheophores) and depositing the heating element, and three steps for the upper face, comprising the steps of depositing the film, the pads and the interdigitized electrodes.
The prior art method described above has inherent limitations, mainly when the four pins come to be soldered to the substrate and to the microelectronic case which may be, for example, a T08 or similar type of package.
Firstly, when the two wires have to be soldered to the heating element after first soldering the two gas sensor film wires, or vice versa, it is necessary to turn the substrate over. This is quite a difficult operation which slows down the soldering process and may lead to damage to the films making up the sensor.
Secondly, the substrate cannot be soldered directly to the case but must be soldered in two steps:
first soldering the wires to the sensor, and
then soldering to the case.
This further slows down the production process.